A micromirror array in which multiple “unit optical elements each reflecting light by means of one or more mirror surfaces” are disposed on a substrate (base) constituting an element surface of an optical element has been developed as an image-forming optical element which image-forms a three-dimensional or two-dimensional object, image and the like. In particular, a micromirror array including a large number of recessed unit optical elements or protruding unit optical elements arranged in an array and each having “two mirror surfaces orthogonal to each other” (a pair of adjacent light reflecting surfaces constituting a right-angled corner; that is, a corner reflector) disposed at right angles or at an angle close to the right angles to this substrate has received attention in recent years because it is simple in structure and it is expected to reduce manufacturing costs (with reference to Patent Literatures 1 and 2).
Among such micromirror arrays, a “dihedral corner reflector array” uses a function such that light incident on one side of the aforementioned array is reflected twice between a pair of light reflecting surfaces constituting each unit optical element (corner reflector) when passing through an element surface (substrate), so that the light reflected twice (passing light) forms an image in a spatial position on the opposite side of the aforementioned array (symmetrical with respect to the plane of the element surface). For example, in the case of a protruding corner reflector array 20 in which a large number of transparent protruding cubes 11 (the ratio of length, width and height is approximately 1:1:1) protruding in the thickness direction of a substrate 2 (element surface P, shown by broken dot-dash line in FIGS. 1 to 4, element surface P is an imaginary plane passing through the center of the thickness of the substrate 2, parallel to the surface of the substrate 2) from one surface of the substrate 2 are arranged in a checkerboard pattern as shown in FIG. 4, at least two surfaces (in this example, a first side surface 11a and a second side surface 11b) among the four side surfaces of such a cube 11 are formed as mirror surfaces (light-reflective side surfaces), so that the aforementioned protruding corner reflector array 20 is capable of forming a mirror image (reversed image) of an object to be projected as an aberration-free real image (erect image) in space thereabove [that is, in space on the side of the eyepoint of an observer (hollow arrow E)], as shown in FIG. 5.
The aforementioned protruding corner reflector array 20 is based on the aforementioned principles. Thus, each cube 11 protruding from the surface of the aforementioned substrate 2 is disposed, with the upper surface (sides of the upper surface) of each cube 11 rotated 45 degrees with respect to the observer so that a right-angled corner (corner 11c) constituting the protruding corner reflector faces toward the front of the observer, as shown in FIGS. 4 and 5. Also, the unit optical elements (cubes 11) constituting the aforementioned protruding corner reflector array 20 are arranged in a diagonal checkerboard pattern as seen from the observer.